Manual cam drum transmission shifters are well-known and commonly used for shifting between the gears of a motorcycle transmission. Typically, the transmission shifter assemblies include a cam drum that is mounted for rotation within the transmission and designed to move a series of gears. Rotation of the drum results in the shifting of the gears in the transmission.
The cam drum typically includes a plurality of equally spaced pins extending axially therefrom. The pins are engaged by a shift pawl that is pivotally connected to a shift lever. The shift pawl engages the pins to rotate the drum when the shift lever is actuated by the motorcycle operator. The shift pawl typically includes spaced-apart opposing teeth or claws. One tooth is operable to rotate the drum in a first or upshift direction, while the opposing tooth is operable to rotate the drum in a second or downshift direction.
During normal shifting, the shift pawl should rotate the drum by indexing through only one of the spaced-apart pins at a time. Sometimes, however, the rotational inertia of the drum during a shift will be great enough to cause the shift pawl to inadvertently index through two spaced-apart pins at once, thereby allowing over-rotation of the drum. Such over-rotation results in the transmission missing a shift, or skipping a gear, which can be hard on the engine. U.S. Pat. Nos. 3,421,384 and 4,455,884 disclose two transmission shifter assemblies having means for preventing the drum from over-rotating during shifting.
The prevention means utilized in U.S. Pat. No. 3,421,384 includes a cam plate fixed to the drum. The cam plate has concavities that correspond to the low speed, high speed and neutral shift positions. The concavities are separated by large projection portions. A swingable stopper is used to prevent over-rotation of the drum and to prevent rotation of the drum from the high speed position directly to the neutral position, thereby bypassing the low speed position. The swingable stopper has a roller that rides in the concavities of the cam plate to prevent the over-rotation or double shifting of the drum.
The prevention means utilized in U.S. Pat. No. 4,455,884 also includes a cam plate and a shift stopper similar to the one taught in the '384 Patent, but further includes a lock mechanism that more positively prevents the shift drum from turning from the high speed shift position directly to the low speed shift position, thereby bypassing the medium shift position. When such an overshift is attempted, a lock piece engages an opening in the surface of the drum and prevents the transmission from shifting directly from the high speed position to the low speed position. When the shift pedal is released, the shift stopper biases the drum back to its high speed position.